The present invention relates generally to a magnetic resonance imaging method and assembly, and, more particularly to a magnetic resonance imaging assembly with a phase controlled surface coil for image correction.
Magnetic Resonance Imaging (MRI) is a well-known medical procedure for obtaining detailed, one, two and three-dimensional images of patients, using the methodology of nuclear magnetic resonance (NMR). MRI is well suited to the visualization of soft tissues and is primarily used for diagnosing disease pathologies and internal injuries.
Typical MRI systems include a super conducting magnet capable of producing a strong, homogenous magnetic field around a patient or portion of the patient; a radio frequency (RF) transmitter and receiver system, including transmitter and receiver coils, also surrounding or impinging upon a portion of the patient; a gradient coil system also surrounding a portion of the patient; and a computer processing/imaging system, receiving the signals from the receiver coil and processing the signals into interpretable data, such as visual images.
MRI systems additionally can utilize surface coils for imaging. Surface coil assemblies are utilized to produce localized magnetic fields to assist in imaging. Often, however, the use of surface coil imaging can result in a left right shading artifact in axial images. This artifact is commonly referred to as a local intensity shift artifact (LISA). At lower field intensities such as 1.5 T (Tesla) the artifacts produced by existing technologies may be barely noticeable. At higher field strengths (3T and above for example) the artifacts may impose significant image quality issues.
The origin of these artifacts stems from an inaccurate assumption regarding the nature of the surface coils. It is often generally assumed that a surface coil produces a linearly polarized magnetic field. However, this may only be the case if the phase shift for the propagation of magnetic energy is everywhere the same from all points on the surface coil to a given point in the region to be imaged. For low frequencies this may be a reasonable approximation, but at higher frequencies this type of assumption begins to break down. Artifacts are further compounded when imaging large water filled objects (such as human subjects). This is a result of the relative dielectric constant than increases about 80 times over that of air. In addition, there are also RF eddy currents induced in the conducting object, which further influences the field.
Surface coils are commonly formed as loops defining a central axis perpendicular to the surface subtended by the loop and traveling through the center of the loop. For MR imaging this axis is commonly orientated perpendicular to the z-axis. The z-axis is taken in the direction of the main magnet field. For MR imaging, it is only the x and y components of the RF coil's magnetic field that are involved in the imaging process. Any variations in design or phase of the various coil segments can result in the x and y components of the magnetization becoming out of phase with each other, depending on their particular location with respect to the coil. The resultant phase difference is typically anti-symmetric with respect to the central axis of the surface coil and can cause the otherwise linear polarization to become elliptically polarized. The direction of the elliptical polarization is different on opposite sides of the central axis of the surface coil.
An elliptical polarized field can be represented by two counter rotating circular polarized fields of differing amplitudes. Because of the different rotation directions, on one side of the central axis the clockwise component is larger than the counter-clockwise component. On the flip-side the reverse is true. In MR imaging it is only one of the circular polarized components of the B1 field that contributes. Therefore, because of the different strengths of the rotating components, there arises asymmetry in the sensitivity of the coil to the MR signal. In practice, therefore, imaging of subjects such as axial images of the spinal region can result in one side of the image becoming brighter than the other.
It would, however, be highly desirable to have a magnetic resonance imaging method and assembly wherein local intensity shift artifact is reduced even under high field strengths. Similarly, it would be highly desirable to have a magnetic resonance imaging method and assembly with controlled reduction in surface coil phase shift to reduce elliptical polarization of the resultant image.